In recent years, a magnetic gear device has attracted attention as means for transmitting power between rotation shafts. A magnetic gear device is constructed by supporting an inner magnet cylinder, an outer magnet cylinder and a magnetic body cylinder, which respectively have a cylindrical shape, in a coaxial manner with the magnetic body cylinder interposed between the magnet cylinders. On the outer circumference of the inner magnet cylinder and on the inner circumference of the outer magnet cylinder, a plurality of rod-shaped magnets extending in the axial direction are respectively arranged uniformly. The magnetic body cylinder is constructed by aligning a plurality of rod-shaped magnetic bodies at equal intervals in the circumferential direction and holding the magnetic bodies with nonmagnetic holders respectively located between magnetic bodies. The number of the uniformly arranged magnets on the inner magnet cylinder is different from the number of the uniformly arranged magnets on the outer magnet cylinder, and the respective magnets are magnetized in a manner such that different magnetic poles lie adjacent to each other in the circumferential direction. Moreover, the number of the juxtaposed magnetic bodies on the magnetic body cylinder is different from the respective numbers of magnets on the magnet cylinders.
In a magnetic gear device, for example, an inner magnet cylinder and an outer magnet cylinder function as rotors supported to be rotatable, a magnetic body cylinder functions as a stator supported to be non-rotatable, and magnetomotive force of one of the inner rotor and the outer rotor is modulated by the magnetic bodies provided on the stator, so that magnetomotive force having a different waveform is given to the other rotor. In such a manner, the magnetic gear device is used for transmitting power (rotational torque) by gear shifting (deceleration, acceleration) between both rotors and rotation shafts which rotate integrally with the rotors.
A magnetic gear device has advantages that contactless power transmission can be achieved, generation of vibration or noise in operation can be suppressed, and lubrication is unnecessary and therefore the maintainability is improved. Moreover, the transmission gear ratio and the rotational direction can be suitably set by selecting the number of the juxtaposed magnets on the inner and outer rotors and the number of the juxtaposed magnetic bodies on the stator. Furthermore, in recent years, high torque density (largest transmission torque per unit size) can be obtained by applying strong magnetic force with a rare earth-iron-boron based magnet or the like. Due to such circumstances, it is eagerly desired to employ a magnetic gear device instead of various gear devices which realize gear shift transmission using a plurality of gears mechanically engaged with each other.
However, torque transmission in a magnetic gear device has a problem that attractive force between magnets on the rotors sequentially changes with relative rotation of the inner rotor and the outer rotor, which function as magnet cylinders, and therefore cogging torque is generated and transmission torque between the rotation shafts periodically fluctuates.
Such torque fluctuation is an unpreferable phenomenon for power transmission means, and reduction of cogging torque is an important issue in practical application of a magnetic gear device.
It is disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2013-47546) that cogging torque can be reduced by providing magnetic bodies (magnetic tooth portions) of a stator, which is configured as a magnetic body cylinder, in a skewed manner with respect to the axial direction of the stator.
Moreover, it is disclosed in Non-Patent Document 1 (Noboru Niguchi and two others, “Study on Transmission Torque Characteristics of a Surface-Permanent-Magnet-Type Magnetic Gear”, IEEJ Transactions on Industry Applications, Vol. 131 No. 3, 2011, pp. 396-402) that magnets of an inner rotor (high-speed rotor), which is configured as a magnet cylinder, are provided in a skewed manner in two steps, so that cogging torque is reduced.